FIELD OF THE INVENTION
The invention relates to a DC/DC converter with bi-directional energy flow.
Such converters can be used for example in motor vehicle technology for the purpose of transferring energy between a starter battery and a fuel cell system.
In the case of an electric vehicle drive fed by fuel cells, the starting operation may necessitate the provision of auxiliary energy with a DC voltage of more than 200 V and a power of approximately 2 kW. Using a step-up converter, the required auxiliary energy can be drawn e.g. from a 12 V starter battery during the starting operation. The starter battery can be subsequently recharged using a step-down converter by the electrical energy that can be supplied by the fuel cell battery with a DC voltage of 250 V, for example.
The use of both a step-up converter and, in addition, of a step-down converter in such an application signifies a considerable outlay.
A DC/DC converter that operates as a step-up converter in a first direction of energy flow and alternately as a step-down converter in the opposite direction is therefore desirable. At the same time, it is necessary to provide DC decoupling of the electric circuits to be connected by the DC/DC converter.
In many applications, particularly in motor vehicle technology, DC/DC converters that operate only as step-down converters but likewise have potential isolation are also required. Such unidirectional DC/DC step-down converters as well as the above-mentioned bi-directional DC/DC converter should be able to be realized particularly compactly in a manner that saves both materials and costs.
One approach to realizing particularly compact converters is provided by Published, Non-Prosecuted German Patent Application DE 195 42 357 A1. A magnetic compound unit and a number of converter circuits can be found therein. The magnetic compound unit can be used to realize converter circuits in which the function of an energy storage inductor with a center tap can be combined together with the function of a potential-isolating transformer in a single inductive component. Various embodiments of the magnetic compound unit are specified in FIGS. 7, 8 and 8a of the Published, Non-Prosecuted German Patent Application DE 195 42 357 A1.
An AC/DC converter circuit which operates as a step-up converter is specified in FIG. 6 of the Published, Non-Prosecuted German Patent Application DE 195 42 357 A1. Although this circuit can be changed into a DC/DC step-up converter in a straightforward manner by omitting the rectifier bridge on the input side, it operates only in one direction of energy flow.
FIG. 11 of the Published, Non-Prosecuted German Patent Application DE 195 42 357 A1 shows a further AC/DC converter circuit, which, depending on the driving, can operate as a step-up or step-down converter, although only ever in one direction of energy flow. This circuit could be embodied as a DC/DC step-down converter in a straightforward manner by omitting the rectifier bridge on the input side. However, the converter has a non-continuous HF output current in the stepping down mode on the output side downstream of the rectification, the AC component of which output current stresses the output capacitor. Therefore, although this configuration is suitable for large output powers, it is not suitable for large output currents.